It is known from the state of the art that a receiving unit might have to synchronize the phase of an available code with the phase of a channel code received via the air interface, before it is able to make use of received signals. The channel code may be in particular a scrambling code, which is used e.g. in WCDMA (wideband code division multiple access) systems for enabling a receiving unit to distinguish between signals from different transmitting units. The synchronization can be performed by comparing a received channel code sample with a respective sample of the available code. In case no correspondence is determined in a correlation procedure, another sample of the available code, which is delayed by a predetermined value versus the first sample, is used for a new comparison.
Such a synchronization may be required for example in a location system which allows to determine the current geographic location of a mobile station within a communication system.
As mentioned in document U.S. Pat. No. 6,191,737 B1, the geographic location of a mobile station may be determined in a communication system based on measurements by the mobile station or based on measurements by a communication network of the communication system. A communication network supporting the determination of a location usually comprises so called local measurement units (LMU), which carry out radio measurements to support one or more positioning methods. An LMU may be realized as a radio receiver positioned at a known location and be able to communicate with radio transmitters of the network via a radio interface, similarly as a mobile station. It can be associated to a serving radio transmitter, but perform air interface measurements as well on signals transmitted by its serving radio transmitter as by neighboring radio transmitters. The radio transmitters of the network can be for instance base stations of a WCDMA network.
In case the location of a mobile station is to be determined based on measurements at the mobile station, first the timing difference between at least two pairs of radio transmitters of the communication network is determined by at least one LMU, if the transmission frames of the different base stations are not perfectly synchronized. The timing difference of the transmissions is determined based on time differences of arrival (TDOA) measured at the LMU and on the knowledge of the location of the LMU and of the radio transmitters. The timing difference can be determined either as relative timing difference (RTD) or as absolute timing difference (ATD) in case the LMU has the absolute time available.
In addition, the mobile station measures the TDOA of signals transmitted by the same pairs of radio transmitters. The TDOAs measured at the mobile station and the known timing difference of the at least two pairs of radio transmitters allow determination of a Geometrical Time Difference (GTD) to two radio transmitters respectively, the GTD being the time difference in transmission between two signals from two different radio transmitters to a single mobile station due to geometry. Since the locations of the radio transmitters are known, the knowledge of the GTDs then enables a determination of the current location of the mobile station.
While mobile stations usually have a code timing information from neighboring base stations, such information is not available for the LMU.
Therefore, the exact code phases of the signals from neighboring base stations have to be found by trial, before the LMU is able to perform RTD or ATD measurements. The trial has to be performed during an initial synchronization or if an already achieved synchronization has been lost again. In addition, the timing of neighboring base stations has to be tracked regularly, in order to avoid a loss of the signal.
In some cases, only short periods of time are available for the synchronization, because it is interrupted due to some reason. For downlink TDOA measurements, idle periods, down link (IPDL) are provided in a pseudorandom order, which prevent a communication in the downlink while allowing the mobile stations and LMUs to make timing measurements spanning 256-1025 chips. During an IPDL, a pilot signal from a neighboring base station comprising a channel code sample can be received at an LMU and stored in a sample RAM for the synchronization with this particular neighboring base station. When the LMU has to search new neighboring base stations while carrying out at the same time measurements for positioning purposes for various neighboring base station, the search algorithm for the new neighboring base stations has to be interrupted frequently.
After each interrupt, the search is started again with a new channel code sample received in another IPDL at the beginning of the corresponding code available in the LMU. In particular in the case of long codes, it may happen that a code synchronization cannot be achieved at all, if the interrupts occur too often. This problem is present, for example, when the currently proposed 3G LMU searches neighboring base stations of the network.